Maritime students spend a certain period of their training on board, depending on their qualifications. Deck officer candidates (deck cadets) must perform an open sea internship on commercial vessels for one year. Deck officers are among to member of crew with important responsibilities to cooperate with the Master during voyage including being involved in navigation and port watches as well as maintenance of the ship and its safety equipment. Deck cadets perform their practical training between academic education periods according to general practice in Turkey. They can be subjected to unusual working hours and rest periods during the mission on the ship. Considering the importance of adequate sleep for productivity, vigilance, sustaining attention and even over-all health and well-being, it is not only sleep quantity but sleep quality is critical. The aim of this study was to investigate of the effect of the long-term onboard training (aprox. 7 months) on the sleep quantity and quality of the maritime students. Data regarding sleep quantity and perceptive quality among the subjects were obtained by using Pittsburg Sleep Quality Index (PSQI). PSQI is a scale providing information on type and violence of sleep disorders and the quality of sleep during the past month. A total of 60 maritime students were asked to fill PSQI before and after onboard training. Demographical information such as age, gender, weight, height and information regarding the internship were also obtained. Of the students 43.3% rated as poor sleeper before the internship and this ratio increased to 73.3% after the internship. A prominent decrease in sleep quality was determined. Sleep duration did not change significantly. This study showed a significant negative effect of onboard training on sleep quality among participants and they did not recover within a month after returning from the sea which indicates potential long term consequencies.